In a typical cellular radio system, user equipment units (also referred to as UEs, wireless terminals, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (also referred to as a network node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with UEs within range of the base stations.
Moreover, a cell area for a base station may be divided into a plurality of sectors surrounding the base station. For example, a base station may service three 120 degree sectors surrounding the base station, and the base station may provide a respective directional transceiver and sector antenna array for each sector.
Multi-antenna techniques can significantly increase capacity, data rates, and/or reliability of a wireless communication system as discussed, for example, by Telatar in “Capacity Of Multi-Antenna Gaussian Channels” (European Transactions On Telecommunications, Vol. 10, pp. 585-595, November 1999). Performance may be improved if both the transmitter and the receiver for a base station sector are equipped with multiple antennas (e.g., an antenna array) to provide a multiple-input multiple-output (MIMO) communication channel(s) for the base station sector. Such systems and/or related techniques are commonly referred to as MIMO. The LTE standard is currently evolving with enhanced MIMO support and MIMO antenna deployments. A spatial multiplexing mode is provided for relatively high data rates in more favorable channel conditions, and a transmit diversity mode is provided for relatively high reliability (at lower data rates) in less favorable channel conditions.
In an uplink from a UE transmitting from an antenna array over a MIMO channel to a base station in the sector, for example, spatial multiplexing (or SM) may allow the simultaneous transmission of multiple symbol streams over the same frequency from the UE antenna array. Thus, multiple symbol streams may be transmitted from the UE to the base station over the same downlink time/frequency resource element (TFRE) to provide an increased data rate.
Similarly, in a uplink from the same UE to the same base station, transmit diversity (e.g., using space-time codes) may allow the simultaneous transmission of the same symbol stream over the same frequency from different antennas of the UE antenna array. Thus, the same symbol stream may be transmitted from different antennas of the UE antenna array to the base station over the same time/frequency resource element (TFRE) to provide increased reliability of reception at the base station due to transmit diversity gain.
The base station and the UE can use adaptive transmission to compensate for dynamic changes in the channel quality therebetween. The adaptive transmission can include channel-dependent scheduling, adaptive MIMO, and adaptive modulation and coding scheme (MCS) that is applied to transmissions. In general, the channel quality varies across time (e.g., frame), frequency (e.g., subcarrier), and space (e.g., antenna port of the spaced apart antennas), which implies the use of time-dependent/frequency-dependent/space-dependent adaptation of the transmissions.
A limiting factor in the ability to effectively adapt such transmissions is the need to reliably measure channel quality. A channel quality measurement is typically based on a reference signal (also referred to as a preamble or a pilot) that is transmitted by the UE to the base station, or vice versa.
Some wireless communication systems adopt several different kinds of reference signals that are not completely aligned in time, frequency, or space. Although certain advantages may be obtained if channel quality could be estimated using heterogeneous reference signals, the lack of time, frequency, and/or space alignment of the reference signals can substantially degrade the reliability of the resulting channel quality estimate.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.